Ion implantation has become a standard accepted technology of industry to dope workpieces such as silicon wafers or glass substrates with impurities in the large scale manufacture of items such as integrated circuits and flat panel displays. Conventional ion implantation systems include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. The ion beam is directed at the surface of the workpiece to implant the workpiece with the dopant element. The energetic ions of the ion beam penetrate the surface of the workpiece so that they are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity. The implantation process is typically performed in a high vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of contamination of the workpiece by airborne particulates.
Ion dose and energy are the two most important variables used to define an implant step. Ion dose relates to the concentration of implanted ions for a given semiconductor material. Typically, high current implanters (generally greater than 10 milliamps (mA) ion beam current are used for high dose implants, while medium current implanters (generally capable up to about 1 mA beam current) are used for lower dose applications. Ion energy is used to control junction depth in semiconductor devices. The energy levels of the ions which make up the ion beam determine the degree of depth of the implanted ions. High energy processes such as those used to form retrograde wells in semiconductor devices require implants of up to a few million electron volts (MeV), while shallow junctions may only demand energies below 1 thousand electron volts (KeV).
The continuing trend to smaller and smaller semiconductor devices requires implanters with ion sources that serve to deliver high beam currents at low energies. The high beam current provides the necessary dosage levels, while the low energy levels permit shallow implants. Source/drain junctions in complementary metal-oxide-semiconductor (CMOS) devices, for example, require such a high current, low energy application.
A typical ion source 10 for obtaining atoms for ionization from a solid form is shown in FIG. 1. The ion source comprises a pair of vaporizers 12 and 14 and an ionization chamber 16. Each of the vaporizers is provided with a crucible 18 in which a solid element or compound is placed and which is heated by a heater coil 20 to vaporize the solid source material. Heater coil leads 22 conduct electrical current to the heater coils and thermocouples 24 provide a temperature feedback mechanism. Air cooling conduit 26 and water-cooling conduit 28 is also provided.
Vaporized source material passes through a nozzle 30, which is secured to the crucible 18 by a graphite nozzle retainer 32, and through vaporizer inlets 34 to the interior of the ionization chamber 16. Alternatively, compressed gas may be fed directly into the ionization chamber by means of a gas inlet 36 via a gas line 38. In either case, the gaseous/vaporized source material is ionized by an arc chamber filament 40 that is heated to thermionically emit electrons.
Conventional ion sources utilize an ionizable dopant gas which is obtained either directly from a source of a compressed gas or indirectly from a solid from which has been vaporized. Typical source elements are boron (B), phosphorous (P), gallium (Ga), indium (In), antimony (Sb), and arsenic (As). Most of these source elements are provided in solid form, except boron, which is typically provided in gaseous form, e.g., as boron trifluoride (BF.sub.3).
In the case of implanting boron trifluoride, a plasma is created which includes singly charged boron (B+) ions. Creating and implanting a sufficiently high dose of boron into a substrate is usually not problematic if the energy level of the beam is not a factor. In low energy applications, however, the beam of boron ions will suffer from a condition known as "beam blow-up", which refers to the tendency for like-charged ions within the ion beam to mutually repel each other. Such mutual repulsion causes the ion beam to expand in diameter during transport, resulting in vignetting of the beam by multiple apertures in the beamline. This severely reduces beam transmission as beam energy is reduced.
Decaborane (B.sub.10 H.sub.14) is a compound which has not heretofore been used as a source of boron for boron implants. The vaporization of decaborane cannot be suitably controlled in the crucible of the ion source of FIG. 1 because decaborane in the solid state has a melting point of about 100.degree. C. Heat generated within the arc chamber 16 will cause the crucible to achieve such a temperature even if the vaporizer heaters are not energized, because the proximity of the solid phase material to the arc chamber results in radiative heating of the material. (The vaporization of phosphorous, on the other hand, can be accurately controlled in the crucible of the ion source of FIG. 1 because it has a melting point of about 400.degree. C.) This prevents the establishment of a moderate temperature (less than 200.degree. C.) thermal equilibrium within the local environment of the source material.
Decaborane, however, is an excellent source of feed material for boron implants because each decaborane molecule (B.sub.10 H.sub.14) when vaporized and ionized can provide a molecular ion comprised of ten boron atoms. Such a source is especially suitable for high dose/low energy implant processes used to create shallow junctions, because a molecular decaborane ion beam can implant ten times the boron dose per unit of current as can a monotomic boron ion beam. In addition, because the decaborane molecule breaks up into individual boron atoms of roughly one-tenth the original beam energy at the workpiece surface, the beam can be transported at ten times the energy of a dose-equivalent monotomic boron ion beam. This feature enables the molecular ion beam to avoid the transmission losses which are typically brought about by low energy ion beam transport.
Accordingly, it is an object of the present invention to provide an ion source for an ion implanter which can accurately and controllably vaporize decaborane, or other suitable implant material, to overcome the deficiencies of known ion sources.